The electrical systems of commercial aircraft are generally grounded to the main structure of the aircraft. In order to accomplish the grounding, a large number of ground wires from the electrical system are connected to ground studs attached to interior surfaces of the main structure at various locations in the aircraft. In a typical large commercial passenger aircraft, there may be as many as 1,000 ground studs each of which has a ground wire attached thereto. During the construction of the aircraft and during certain types of maintenance activity, it is essential to ensure that at least the most critical of these ground connections form good ground paths from the electrical system to the main structure. Without a good ground path, a lightening strike or an electrical fault would not be properly absorbed into the main structure and damage or failure of the electrical system could result.
Methods and apparatus currently in use for testing the ground connections have a number of problems associated with them. Many of the more troublesome of these problems are a result of the fact that the structural member to which the ground stud is attached has a protective coating thereon. This coating usually consists of a primer and paint having a thickness in the order of 1.8 mils. The coating must be completely and consistently penetrated in order to test the resistance between the structural member and the ground stud. However, it is of course desirable to minimize any damage to the coating caused by the testing procedure in order to preserve the corrosion protection function of the coating.
Devices currently in use to make contact with the conductive structural member do not consistently fully penetrate the protective coating and, thus, do not reliably result in accurate readings. When additional force is applied to the device in an effort to ensure complete penetration of the coating, the result is often damage to the device and/or damage to the coating that is sufficiently extensive to require repair of the coating following the testing procedure. The degree of the penetration provided by known devices is user dependent. This dependence on the individual operator results in inconsistent and unreliable readings. In order to avoid the problems of damage to the coating and inconsistent and/or unreliable readings, in some situations the structural member has been spot cleaned before the testing operation in order to ensure a good reliable electrical path. This procedure increases the reliability and accuracy of the testing, but has the serious disadvantage of requiring recoating of the cleaned area following the testing. Such recoating is time consuming and makes the whole testing procedure generally more complicated and more expensive to carry out. The expense of existing systems is further increased by the lack of durability and the relatively short useful life of known devices.
There are a large number of known electrical testing devices having spring biased point contact probes. Such devices are disclosed in U.S. Pat. Nos. 2,659,862, granted Nov. 17, 1953, to N. G. Branson; No. 3,229,200, granted Jan. 11, 1966, to C. C. Rayburn; No. 3,644,829, granted Feb. 22, 1972, to J. Y. Chan et al; No. 3,944,922, granted Mar. 16, 1976, to R. N. C. Chambers et al; No. 3,996,516, granted Dec. 7, 1976, to E. Luther; No. 4,335,350, granted June 15, 1982, to J. T. C. Chen; and No. 4,394,620, granted July 19, 1983, to A. R. Montalto et al. Such devices are also disclosed in British Pat. No. 843,870, published Aug. 10, 1960, and disclosing the invention of S. R. Richardson; IBM Technical Disclosure Bulletin, Volume 14, No. 2, dated July 1971, and entitled "Twin-Lead Contact Probe"; IBM Technical Disclosure Bulletin, Volume 18, No. 9, dated Feb. 1976, and entitled "Plunger Contact Assembly"; and "The Measurement of the Thickness of Metal Walls From One Surface Only, By An Electrical Method", an article by B. M. Thornton and W. M. Thornton on pages 349 and 356 of "Proceedings: The Institution of Mechanical Engineers (London)", Volume 140, Oct.-Dec. 1938.
U.S. Pat. Nos. 2,763,935, granted Sept. 25, 1956, to R. M. Whaley et al; No. 2,885,648, granted May 5, 1959, to H. H. King; and No. 4,104,579, granted Aug. 1, 1978, to M. C. McIvor each disclose electrical testing apparatus having point probes that are apparently fixed. Such apparatus is also shown in West German Pat. No. 936,480, dated 1955, and disclosing an invention by Karl Zangl. West German Pat. No. 2,919,541, dated 1979, discloses an invention by Viktor Vanicek consisting of an electrical test instrument having point probes that are mounted on a slidable head so that they may be driven into wood by an impact device to test the moisture content of the wood.
In the testing procedure disclosed by Luther, a lacquered printed circuit is contacted by probes and then the circuit is subjected to ultrasonic vibrations to remove excess coating, impurities, and oxides along the points of contact to ensure good contact. Montalto et al disclose a test device in which the test probes are introduced into apertures to contact test lands within the apertures. The probes are biased outwardly by springs that are described as exerting force sufficient to penetrate any nonconductive coatings on the test lands. The device described in the article by the Thorntons includes "spring-controlled rotating points".
U.S. Pat. No. 2,323,925, granted July 13, 1943, to L. J. Markwardt discloses a testing tool for testing the hardness of nonmetallic materials. The tool includes two pointed pins that penetrate the material being tested. In one embodiment of the tool, an off-center punch mechanism is included to provide an impact force of a constant magnitude to drive the pins into the material. The displacement or amount of penetration of the pins is then measured to determine the degree of hardness. Another embodiment of the tool permits hardness to be measured as a function of the amount of force required for achieving a predetermined amount of penetration.
The above patents and the prior art that is discussed and/or cited therein should be studied for the purpose of putting the present invention into proper perspective relative to the prior art.